(1) Field of the Invention
The present invention relates to an inspection station and a printing station, in particular for a screen printing apparatus, a screen printing apparatus incorporating the same, and a method of screen printing deposits of material onto workpieces, in particular substrates, through a printing screen, often referred to as a stencil.
The present invention finds particular application in the electronics field in printing deposits of solder paste onto printed circuit boards prior to the subsequent placement of electronic components.
(2) Description of the Related Art
In printing material onto workpieces, particularly solder paste onto contact pads of a printed circuit board, it is important to accurately align the workpiece with the printing screen through which material is to be deposited.
Existing screen printing apparatus include an imaging system which utilizes video camera units and image processing software to determine any misalignment of a workpiece and the printing screen, and hence the relative positional correction of the workpiece and the printing screen which is required to accurately align the workpiece with the printing screen prior to printing material onto the workpiece. In such an imaging system, the camera units are introduced between the workpiece and the printing screen to image features at at least two widely-spaced positions on both the workpiece and the printing screen. From this image data, the misalignment can be determined, and the workpiece moved relative to the printing screen in order to correctly align the workpiece and the printing screen.
Although such an imaging system functions perfectly satisfactorily, this system does suffer from a number of disadvantages, particularly for the implementation of high-speed screen printing apparatus. Significantly, the cycle time of screen printing apparatus which utilizes such an imaging system is relatively long. There are time delays associated with introducing and withdrawing the camera units in acquiring the image data, and also with the requirement to close the necessarily large spacing between the workpiece and the printing screen to the required spacing for screen printing. In requiring the location of the camera units between the workpiece and the printing screen, the workpiece has necessarily to be spaced a large distance from the printing screen in order to be able to physically fit the camera units therebetween. This spacing is typically at least 80 mm. In addition, when single camera units are used to image features on the workpiece and the printing screen, there is a further time delay associated with moving the camera units to at least a second position and acquiring the image data at those positions. These time delays contribute to the cycle time and hence limit the throughput of the screen printing apparatus. Furthermore, misalignment can be re-introduced when closing the large spacing between the workpiece and the printing screen.
U.S. Pat. No. 5,943,089 discloses an alternative imaging system which is configured such as to be disposed beneath the printing screen and a workpiece to be printed, and thereby image features on lower surfaces of the printing screen and the workpiece. This imaging system avoids the need for the introduction and withdrawal of camera units from between the printing screen and the workpiece in acquiring the image data, and, as such, does not require a large spacing between the workpiece and the printing screen.